1. Field of the Invention
Embodiments of the present invention relate generally to time and frequency alignment systems operating over packet-switched communications networks and, more specifically, to methods and apparatus for distributing location information in addition to precision time transfer.
2. Description of Related Prior Art
It has been recognized that by establishing the geographic location of a mobile telephone user, location-based services can be provided, thereby increasing the potential average revenue per user. It is also a federal mandate (in the USA) that, for emergency services, the geographic location of the origin of an emergency (i.e. E911) call be established with a prescribed level of accuracy. GPS-equipped mobile devices can establish their own geographic location if there is a good and unobstructed view of the sky. In other cases the location of the mobile station is established relative to the location of the serving base-station. That is, it is advantageous to establish the geographic location of the base-stations.
Outdoor base-stations can be equipped with GPS antenna/receiver functionality and thereby establish their location autonomously. In the case of smaller, typically indoor-mounted, base-stations, the option of self-positioning via GPS is not a viable option for reasons of cost and/or visibility of the sky. Determining the location of a base-station that does not have GPS functionality is done by manually surveying the location where the base-station is installed prior to deployment.
Such pre-deployment surveying does not satisfactorily address the case where the base-station itself can be moved. Pre-deployment surveying is also not appropriate in the case where the base-station device is purchased and installed by the end-user.
The provider of mobile communication services requires knowledge of the location of the base-station for variety of reasons such as billing and often the service contract pre-supposes a deployment location. Consequently it will be advantageous to ascertain the location of a deployed base-station, albeit approximately, in order to deliver mobile communication services.
The conventional methodology for distributing timing to base-stations is depicted in FIG. 1. The various base-stations (BS-x) 120 are connected back into the service provider network via communication links 160. The provider has at least one location that can operate as a master clock (MCLK) 110 that represents the timing reference for the base-stations in the (sub)network that home in to the master clock. The master clock is usually associated with a Radio Network Controller (RNC) or a Base-Station Controller (BSC) or even the Mobile Switching Office (MSO). Legacy mobile telephony networks often used TDM links to implement the backhaul from the base-stations and these TDM links (e.g. T1/E1 or SONET/SDH) were suitable for carrying a (frequency synchronization reference signal.
It is increasingly common for the backhaul network to be replaced with a packet-switched network wherein the physical layer could be implemented by a wide variety of technologies including Ethernet (typically over optical fiber), microwave, ADSL/VDSL, and coax (cable-TV derivative). The timing information 270 in this case is delivered in the form of a packet flow 260. The technologies used for delivering timing in this situation are packet-based including precision time protocol (PTP) and/or network time protocol (NTP). Whereas legacy mobile telephony required simply a frequency reference, more recent advances require a time/phase reference (as well as frequency). Two-way methods such as PTP and NTP are required to support this requirement. As depicted in FIG. 2, timing information is delivered from the master base station 110 clock 215 to the slave clock 225 in the base station 120 over a packet network 250 wherein the processing elements 216 and 226 exchange packets via a flow 260.
Whereas FIG. 1 and FIG. 2 depict terrestrial methods for distributing time/frequency from the central location to the base-stations, they do not teach how the base-station can assess its own geographical location. In FIG. 3 a third approach is shown wherein all the elements (base-stations and master clock) derive their timing from a common source, namely a Global Navigation Satellite System (GNSS) 310 (the most commonly quoted example of a GNSS is the global positioning system (GPS) operated by the US Government). The satellite signal 330 is received by the terrestrial elements and by synchronizing to GNSS the terrestrial elements are indirectly aligning themselves in time/frequency. One advantage of the GNSS signal is that it provides not just timing but enables the receivers to establish their own geographical location(s). However, due to constraints such as cost and deployment location considerations, base-stations with GPS/GNSS functionality are usually macro-base-stations deployed outdoors; smaller base-stations (such as micro-, pico-, and femto-base-stations) are often deployed indoors and without a clear view of the sky.